IntroductionThis document describes the recommendation for die sintering developed by STMicroelectronics with Ag finishing.

Recommendation for die sintering with Ag finishing

TN1340

Technical note

TN1340 - Rev 1 - September 2020For further information contact your local STMicroelectronics sales office.

www.st.com

1 General description

1.1 DA of SiC dice with sintering

Our new SiC dice generation have been conceived for automotive domain, they are thought to be mountedthrough non-conventional DA techniques. The new SiC dice are designed to be assembled through sintering bothon the bottom and on top for clip sintering. The dice and clip sintering emphasizes the exceptional electrical andthermal performance of the SiC as well as superior reliability under temperature swing.Figure 1. Cross section: sketch of the AMB and die sintered and Figure 2. Cross section: optical image of theAMB, die and clip sintered shows typical stack configuration of die sintered on substrate with clip sintered on topof the dice.

Figure 1. Cross section: sketch of the AMB and die sintered

Clip

D ice

Clip

Ag layer

GADG070820201217SA

AMB

Clip

Figure 2. Cross section: optical image of the AMB, die and clip sintered

ClipClip

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2 Sintering requirements for substrate and clip

2.1 Ag surface for sintering

The contact area of the dice, the substrate and/or the clip must be free of particle and damage as well as anyother contamination. It is very important to avoid Ag finishing oxidation of all the surface that must be put incontact with sinter media because the oxide inhibits the coalescence of the Ag particle with the Ag grain of thesurface. To prevent the oxidation transportation must be done under vacuum and storage must be donepreferable under vacuum in dry box with inert atmosphere. In case of reuse of the parts after the box is opened, itis suggested a light plasma cleaning in order to reactivate the surface of the silver.

2.2 Substrate and clip surface requirements

Substrate and clip roughness: Rz ≤ 10 μm , substrate flatness: ≤ 10 μm (reference length 20 mm) .For a reliable sintering process, the substrate and clip needs to be coated in silver in order to ensure most reliablesintering joint. Typical stack and materials for clip and substrate are reported in the pictures below:Figure 3. Cross section: layers details of clip sintered and Figure 4. Cross section: details of die sintered on AMBshow the cross sections of a typical stack configuration of clip sintered on top of the die and the die sintered ontop of the substrate.

Figure 3. Cross section: layers details of clip sintered

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AClip material: C192 – KFC withselective Ag plating electroless 3-9 µm

B Ag Sintering layer 20-25 µmC Top of the die with 0.5 µm Ag finishing

B

C

Figure 4. Cross section: details of die sintered on AMB

GADG100820201029SA

1Bottom of Dice: SiC dice with 0.5 µmAg finishing

2 Ag Sintering layer 15-22 µm3 Substrate: copper OFCu with 0.1-0.6 µm

Ag finishingfinishing

the die

1

2

3

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3 Sintering media selection and deposition suggestions

3.1 Sintering media recommendation

Our SiC dice developed for Ag sintering DA have been internally qualified for the use of the Ag paste and Ag thefilm. The supplier qualified are: Heraeus for the paste and Alpha for the film. The paste can be printed ordispensed up to 110 microns thick, instead film thickness through DTF (1) processing is 65 microns. After thesintering the final bond line will be roughly 30% of the printed/dispensed paste or the attached film. The finalthickness will depend on customer applications.

3.2 Sintering media deposition

The application of the paste or the film on the dice have different assembly processes: the paste can be appliedthrough screen printing or it can be dispensed in wet condition, before sintering it needs to be submitted to drying(for the conditions, it is necessary to refer to the supplier recommendations), the film applied with DTF (1) no needdrying before sintering. Optionally in the clip the paste or the film can be pre-applied from the supplier reducingthe process steps during the assembly of the product at the final customer. The sintering paste can be stencilprinted through Dek , Ekra or similar automatic equipment or dispensed through Musashi, Asymtek , Nordson orsimilar, the film can be transferred with automatic an pick and place machine equipped with a heated jig and pickup tool like Datacon, ASM or similar. Below are reported typical DA assembly flows for paste and film and relatedequipments:It is recommended to dry in Nitrogen atmosphere with standard ovens commonly used in the market with typicaldrying times 20 minutes at 120 ˚C, for more details it is necessary to refer to the Ag paste supplier.

Figure 5. Typical equipments for die sintering with paste

ClipClip

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Figure 6. Typical equipments for die sintering with film

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Argomax® Die Lamination DTF process is recommended at 150 ˚C substrate temperature.

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4 Sintering process

4.1 A brief recap of what is silver sintering

The silver sintering became an excellent alternative to soldering, especially for high temperature applications.Indeed, the increase in operating temperature requires new soldering alloys with even higher melting points.Silver sintering, on the contrary, is a solution which only requires a moderate (<300 °C) process temperature andpressure. Sintering is a combined surface, volume, and grain boundary diffusion process. Several processes(densification, grain growth, pore growth / coarsening) take place in parallel.Silver sintering exploits an Ag thermal conductivity >200 W/mK, sintering joint thickness in the range of 50 to 100μm, and even lower, is possible, the Ag melting temperature is 961 °C.Resulting advantages are:• Improved heat dissipation from semiconductor devices• increased reliability

Figure 7. Ag sintering process flow

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The silver media, the paste / film is basically silver powder mixed with additives (thinner, binder, dispersant). Thesilver powder particle size ranges from few nanometers up to micrometers. Reducing the particle size results inhigher specific surface (more particle surface per powder volume), thus accelerates the diffusion process. Interms pastes , typically the additives / organic residues are removed by applying heat treatment prior to finalsintering process in the film there is no need for drying treatment.Figure 8. SEM picture of sintering media before (a) and after (b) pressure sintering SEM picture of sintering mediabefore (a) and after (b) pressure sintering.

Figure 8. SEM picture of sintering media before (a) and after (b) pressure sintering

ClipClip

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Figure 9. FIB pictures of sintering layer porosity. Lower closed porosity leads to better reliability FIB picture ofsintering layer porosity. Lower closed porosity leads to better reliability.

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Figure 9. FIB pictures of sintering layer porosity. Lower closed porosity leads to better reliability

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4.2 Precautions and preparatory process before real sintering

The sintering process is a slow but powerful process. A very strong force is delivered onto dice. A direct metalcontact to the top side of the dice may damage the passivation and structure underneath the surface if dice areshowing any tilting/misplacing. In order to reduce likelihood to damage the dice, ST recommends to deliversintering pressure through a pressing elements that has the same dimension of the dice putting between steelpressing element and the top of the dice surface a Teflon foil. Especially if many dice are sintered simultaneously,the foil would help equalizing natural slight thickness differences. The Teflon foil may be selected among severalthicknesses selection. The foil has got a standard compression rate of 30% and strongly helps to evenly deliverpressure. STMicroelectronics suggests to pick the thickness that best suits the customer's requirement, even arange from 50 to 75 μm is preferable.

Figure 10. Schematic of pressure sintering of the die with individual insert

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Metal insert

Teflon foil

substrate

Sinter layer

dice

Metal insert

4.3 Sintering process

After the paste deposition or film lamination the sintering process is the same for both kind media. In the order toachieve best sintering results, parts should be preheated in order to generate the lowest stress during realsintering process. Here below basic recommendations for dice sintering process:1. Raise dice parts temperature from room temperature to 80 ˚C along 2 minutes.2. Apply sintering pressure in the range of 15 - 20 MPa and sintering temperature (top and bottom plate) in the

range from 230 - 250 ˚C.3. Hold pressure and temperature in the range of 180 - 240 seconds.4. Return to room temperature gradually through a cooling plate.

Figure 11. Die sintering process details

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.

Pre heating80˚C along 2 minutes Sintering Pressure:15 - 20MPaTemperature:230 - 250 ˚CTime180 - 240 sec

Cooling down

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5 How to make sure dice is correctly sintered

Die is best sintered when the shear force to remove it from the substrate is in the range of 55 - 60 N/mm². Thefailure mode needs to be cohesive. It means the sinter media leftover Ag layer must to be found over dicebackside/top as well as substrate or clip sintering plane. No native Ag layer both on the substrate/clip or dice mustbe peeled off during test. The shear test is the best method, destructive though, to deem good sinter quality. It isrecommended to make sure the clamping is effective during tests as the result may shift.

Figure 12. Shear test: Ag layer is visible in the AMB area where the dice was

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As an alternative to the shear test there is the bending test. It consists to flex around a cylinder of 20 mm diameterthe substrate where the dice are sintered, if the sintering is well done the dice remain attached even if it is brokenand follow the shape of the substrate. On the contrary if the dice are not well sintered it will be detached from thesubstrate.Figure 13. Bending test schematic shows AMB after bending test, on the left the correct failure mode shows thedice broken but still attached in the baseplate whereas on the left the dice are detached.

Figure 13. Bending test schematic

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Figure 14. AMB and die after bending test

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Another control method is C-SAM analysis through the Ultrasonic Microscope. This is a non destructive methoduseful for integration during the process assembly flow.Figure 15. C-SAM good shape and Figure 16. C-SAM bad shape show C-SAM image on the left good results onthe right bad results.

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Figure 15. C-SAM good shape

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Figure 16. C-SAM bad shape

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6 Dice handling

6.1 ESD protective measures

Semiconductors are normally Electrostatic Discharge Sensitive Devices (ESDS) requiring specific precautionarymeasures regarding handling and processing. Static discharges caused by human touch or by processing toolsmay cause high-current and/or high-voltage pulses, which may damage or even destroy sensitive semiconductorstructures. On the other hand, Integrated Circuits (ICs) may also be charged by static during processing. Ifdischarging takes place too quickly (“hard” discharge), it may cause peak loads and damages, too. ESDprotective measures must therefore prevent any contact with charged parts as well as charging of the ICs.Protective measures against ESD include proper procedures for the handling, processing, and the packing ofESDS. A few hints are provided below on handling and processing.

6.1.1 ESD protective measures in the workplace• Standard marking of ESD-protected areas• Access controls, with wrist strap and footwear testers• Air conditioning• Dissipative and grounded floor• Dissipative and grounded working and storage areas• Dissipative chairs• Ground bonding point for wrist strap• Trolleys with dissipative surfaces and wheels• Suitable shipping and storage containers• No sources of electrostatic fields

6.1.2 Equipment for personal• Dissipative/conductive footwear or heel straps• Suitable garments made of fabrics that do not generate excessive static electricity• Wrist strap with safety resistor• Volume conductive gloves or finger cots

6.1.3 Production installations and processing tools• Machine and tool parts made of dissipative or metallic materials• No materials having thin insulating layers for sliding tracks• All parts reliably connected to ground potential• No potential difference between individual machine and tool parts• No sources of electrostatic fields

Our recommendations are based on the internationally applicable standards IEC 61340-5-1 and ANSI/ESDS2020.

6.2 Packing of components

Please refer to product and package specifications and our sales department to get information about whatpackaging is available for a given product.Generally the following list of standards dealing with packing should be considered if applicable for a givenpackage and packing:• IEC 60286-4 Packaging of components for automatic handling - Part 4:Stick magazines for dual-in-line

packages

IEC 60286-5 Packaging of components for automatic handling - Part 5:Matrix trays.

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6.3 Storage and transportation conditions

Improper transportation and unsuitable storage of components can lead to a number of problems duringsubsequent processing, such as poor solderability, delamination, and package cracking effects.These relevant standards should be taken into account as appropriate:• IEC 60721-3-0 Classification of environmental conditions: Part 3: Classification of groups of environmental

parameters and their severities; introduction• IEC 60721-3-1 Classification of environmental conditions: Part 3: Classification of groups of environmental

parameters and their severities; Section 1: Storage• IEC 60721-3-2 Classification of environmental conditions: Part 3: Classification of groups of environmental

parameters and their severities; Section 2: Transportation• IEC 61760-2 Surface mounting technology - Part 2: Transportation and storage conditions of surface

mounting devices (SMD) - Application guide• IEC 62258-3 Semiconductor Die Products - Part 3: Recommendations for good practice in handling, packing

and storage

ISO 14644-1 Clean rooms and associated controlled environments Part 1: Classification of airborne particulates

Table 1. General storage conditions - overview

Product Condition for storing

Wafer/die N2 or MBB(1) (IEC 62258-3)

Component - not moisture sensitive 1K2 (IEC 60721-3-1)

1. MBB= moisture barrier bag

Maximum storage timeThe conditions to be complied with in order to ensure problem-free processing of active and passive componentsare described in standard IEC 61760-2.Internet links to standards institutes• American National Standards Institute (ANSI)• Electronics Industries Alliance (EIA)

Association Connecting Electronics Industries (IPC)

6.4 Handling damage and contamination

Any mechanical damage during automatic or manual handling of components (in or out of the componentpacking) that may harm dice has to be avoided. In particular, unintentional scratch, on surface may cause shortcircuit and can result in electrical malfunction.Along with other factors, any contamination applied to a component or packing or exposure to uncontrolledatmosphere may cause:• Sintering problems (pollution, Ag oxidation etc.)• Corrosion• Electrical shorts (due to conductive particles)

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Revision history

Table 2. Document revision history

Date Revision Changes

17-Sep-2020 1 Initial release.

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Contents

1 General description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

1.1 DA of SiC dice with sintering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Sintering requirements for substrate and clip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

2.1 Ag surface for sintering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 Substrate and clip surface requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3 Sintering media selection and deposition suggestions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

3.1 Sintering media recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.2 Sintering media deposition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

4 Sintering process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

4.1 A brief recap of what is silver sintering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4.2 Precautions and preparatory process before real sintering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4.3 Sintering process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

5 How to make sure dice is correctly sintered . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

6 Dice handling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

6.1 ESD protective measures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

6.1.1 ESD protective measures in the workplace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

6.1.2 Equipment for personal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

6.1.3 Production installations and processing tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

6.2 Packing of components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

6.3 Storage and transportation conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6.4 Handling damage and contamination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

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IMPORTANT NOTICE – PLEASE READ CAREFULLY

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Information in this document supersedes and replaces information previously supplied in any prior versions of this document.

© 2020 STMicroelectronics – All rights reserved

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